The present invention relates to a high strength alumina porcelain and its formation by wet processing, particularly for use as electrical porcelain insulators.
Alumina has been used in electrical porcelain bodies to provide a high mechanical and electrical strength. Such porcelain is used to mechanically hold up a high tension line or piece of electrical apparatus and to provide adequate insulation. It is recognized that a certain increase of strength of such bodies is obtained by increasing the alumina content up to 40% alumina. In a book entitled "Current Development in The Whiteware Industry", George A. Kirkendale, Editor, University of New York College of Ceramics at Alfred University, Alfred, N.Y., (1966), beginning at page 29, it is stated that although there is an increase in body strength with increasing amounts of alumina, a range of 30 to 40% alumina appears the most reasonable. This is due to the problems inherent in providing sufficient plasticity to porcelains of high alumina content to obtain a workable aqueous mass for wet forming a ceramic body prior to firing. This is important because the shapes of electrical porcelain bodies may be relatively complex.
Examples of ceramic bodies including 40% calcined alumina are disclosed in U.S. Pat. No. 2,898,217. Such product (including 20% nepheline syenite, 2% magnesium oxide, and 38% clay) has a modulus of rupture of 26,000 psi. The patent relates to the use of nepheline syenite to reduce the quartz content of the charged ceramic mix to thereby increase the strength of the final product. The patent indicates that the process would not be applicable to alumina contents in excess of 45%. Presumably, this is because at higher alumina content, the ceramic mix would not be suitable for wet processing.
Very high strength alumina porcelains are known for use as electrical insulators. For example, an article by W. E. Blodgett entitled "High Strength Alumina Porcelains", The American Ceramic Society Bulletin, Vol. 40, No. 2 (1961), discloses ceramic bodies including 65% alumina. However, the total disclosed clay content is only about 20%, insufficient to provide the requisite workability for wet processing. Thus, the ceramic body was formed by dry pressing. This technique is not suitable for commercial production of the common electrical insulator shapes which usually include undercut shed designs and variable wall thicknesses. Even if such dry pressing techniques were developed for such shapes, the process would be very costly.